1. Field of the Invention
This invention relates to a brushless direct current (BLDC) motor having a plurality of magnets in order to increase a torque of the motor and to reduce a noise of a device driven by such a motor.
2. Description of the Conventional Art
A brushless direct current (BLDC) motor is often used as a driving motor of an optical scanning apparatus or a video tape recorder head scanner.
In the brushless direct current motor as shown in FIG. 1, an oil-less beating 1 is mounted at a predetermined position spaced from an upper surface of a washer 2.
The washer 2 is integrally formed with a bearing bush 3 by a caulking process such as a rivetting process. Also, the beating bush 3 is engaged with a main base 4 by a caulking process. A motor bush 5, which is part of the rotor, is inserted into a motor shaft 6 and is integrally engaged with a rotor yoke 7 by a caulking process.
A first magnet 8 is mounted on an upper surface of the rotor yoke 7. The rotor yoke 7 is made of a magnetic material and is spaced from the main base 4 when the motor shaft 6 is inserted into the oil-less beating 1.
Also, a back yoke 9 is inserted onto the motor bush 5 and is fixed in place by means of the magnetic force of the first magnet 8. A fine pattern coil 10 is engaged and fixed on the main base 4 by screws (not shown) to be disposed between the first magnet 8 and the back yoke 9.
The motor shown in FIG. 1 has an axial air gap between the first magnet 8 and the fine pattern coil 10. The magnetic flux of the driving magnetic poles of the magnet 4 interacts with the fine pattern coil 10. When drive currents are supplied to the fine pattern coil 10, they generate driving magnetic flux which interact with the magnetic flux of the driving magnetic poles to rotate the magnet 4 and hence the rotor yoke 7 and motor bush 5.
When a conventional BLDC motor is driven in such an arrangement, a belt 11 engaged with the motor bush 5 is driven and thus a rotation force of the motor is transferred to a mechanical mechanism (not shown). At this time, the magnet flux created from the first magnet 8 is magnetized along the direction indicated by the arrows as shown in FIG. 1.
The magnetic flux which leaks in a downward direction (direction of the arrows in FIG. 1) from the magnet 8, acts as an attractive engaging force to the main base 4 and prevents any separation of the rotor unit from the oil-less bearings 1 even when the motor is inverted or overturned. There are problems if the attractive engaging force is too weak, hence the rotor unit will come off due to its own weight, and if the attractive engaging force is too strong, in other words, when the leakage magnetic flux is high, then magnetic loss, which is proportional to the squared value of the increased portion of the leakage magnetic flux, will increase.
As explained above, when a conventional BLDC motor is inverted or overturned, possible dislodgement of the motor unit is prevented by the attractive engaging force which pulls together the first magnet 8 and the main base 4, through the use of the magnetic leakage flux. The thickness of the rotor yoke 7 and the size of the magnet 8 are determined according to the degree of attractive engaging force between the first magnet 8 and the main base 4, while power consumption, the relation to magnetic flux and noise reduction in addition to many other restrictive requirements which need to be considered in motor design are all regarded as disadvantages.